US 6765671 B2 Abstract A method for automating measurement of an optical property of a sample includes selecting a measurement aperture around a reference point on the sample (
38), generating a set of grid nodes that fall within the measurement aperture (68), calculating the radial distance of each node with respect to a reference point within the measurement aperture, and calculating the angular position of each node with respect to the vertical. The method also includes moving a light source (32) and a light detector along the vertical and rotating the sample to measurement positions in which the light source and the light detector are aligned with one of the nodes in the measurement aperture, and measuring the optical property at the measurement position by energizing the light source and interrogating the detector. The calculated radial distances and angular positions are used to control positioning of the light source and the light detector and rotation of the sample.Claims(17) 1. A method for automating measurement of an optical property of a sample, comprising:
selecting a measurement aperture around a reference point on the sample;
generating a set of grid nodes that fall within the measurement aperture;
calculating the radial distance of each node with respect to the reference point;
calculating the angular position of each node with respect to vertical;
moving a light source and a light detector along the vertical and rotating the sample to measurement positions in which the light source and the light detector are aligned with one of the nodes in the measurement aperture, wherein the calculated radial distances and angular positions are used to control positioning of the light source and the light detector and rotation of the sample; and
measuring the optical property at the measurement position by energizing the light source and interrogating the detector.
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13. A computer readable storage medium containing an executable program for use in automating measurement of an optical property, the executable program comprising instructions that when executed by a computer enable the computer to:
select a measurement aperture around a reference point on the sample;
generate a set of grid nodes that fall within the measurement aperture;
calculate the radial distance of each node with respect to the reference point;
calculate the angular position of each node with respect to the vertical;
generate signals to move a light source and a light detector along the vertical and to rotate the sample to measurement positions in which the light source and the light detector are aligned with one of the nodes in the measurement aperture, wherein the signals are generated in accordance with the calculated radial distances and angular positions; and
record measurements made at the measurement positions.
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Description This application claims the benefit of priority of U.S. application Ser. No. 09/458,561, filed Dec. 9, 1999, entitled Automated System For Measurement Of An Optical Property, of Richard S. Priestley and U.S. Provisional Application Serial No. 60/204,405, filed May 16, 2000, entitled Automated System For Measurement Of An Optical Property, of Richard S. Priestley, which is hereby incorporated by reference. 1. Technical Field The invention relates generally to systems for measuring birefringence or other optical property, e.g., transmission, of a sample of material. 2. Background Art Birefringence, or double refraction, is a phenomenon that occurs in materials characterized by two indices of refraction. Typically, birefringent materials are optically anisotropic substances, e.g., calcite and quartz, although some isotropic materials such as glass and plastic become birefringent when subjected to stress. When a beam of light enters a birefringent material, the beam splits into two polarized rays each traveling at a different velocity, corresponding to a different index of refraction. One ray, called an ordinary ray, is characterized by an index of refraction that is the same in all directions. The second ray, called an extraordinary ray, travels with different speeds in different directions and hence is characterized by an index of refraction that varies with the direction of propagation. If the light entering the birefringent material is unpolarized or linearly polarized, the ordinary and extraordinary rays will have the same velocity along one direction, called the optic axis. The ordinary and extraordinary rays recombine upon exiting the material. Birefringent materials can change the polarization state of a light passing through them. Therefore, the ability to accurately determine the birefringence of a sample is important, especial y in high performance optics, e.g., ophthalmic lenses, laser optics, and optical fiber, where a change in the polarization state of light can cause dramatic changes in optical performance. When linearly polarized light passes through a birefringent sample, the sample rotates the direction of polarization through some angle. By measuring this angle of rotation, the birefringence of the sample, i.e., the difference between the highest and lowest indices of refraction of the sample, can be determined. Typically, the sample is placed between two crossed linear polarizers. The birefringence at a given point about the cross section of the sample is then determined by measuring the angular position, with respect to the first linear polarizer, at which the light emerging from the sample is extinguished as it passes through the second linear polarizer. Various other methods are known for determining birefringence. One example of a known method is disclosed in U.S. Pat. No. 5,257,092 issued to Noguchi el al. As shown in FIG. 1, an optical source unit Another example of a method for measuring birefringence is disclosed in U.S. Pat. No. 5,587,793 issued to Nakai el al. As illustrated in FIG. 2, a sample The birefringence of the sample may vary from location to location across the sample. Thus, in order to describe the birefringence of a sample, birefringence at a number of points along or distributed on the surface of the sample is measured. One procedure used in industry includes taking a measurement at one position on the cross section of a sample and then manually moving the sample e.g., by using a lab jack, so that the measurement is made at another test point on the cross section. The measurements are repeated at numerous test points about the cross section of the sample to generate a birefringence map. Because mapping requires a large number of points, mapping the sample manually is a difficult and time-consuming task. In some cases, the actual birefringence measurement is also performed manually, with the operator having to determine the actual angle of light extinction. Therefore, the accuracy of these measurements can vary from operator to operator. The invention is a method for automating measurement of an optical property of a sample. The method comprises selecting a measurement aperture around a reference point on the sample, generating a set of grid nodes that fall within measurement aperture, calculating the radial distance of each node with respect to a reference point within the measurement aperture, and calculating the angular position of each node with respect to the vertical. The method further includes calculating the angular position of each node with respect to the vertical and moving a light source and a light detector along the vertical and rotating the sample to measurement positions within the measurement aperture. The calculated radial distances and angular positions are used to control positioning of the light source and the light detector and rotation of the sample. The optical property is measured at the measurement position by energizing the light source and interrogating the detector. FIG. 1 shows a prior art system for determining birefringence of a material. FIG. 2 shows another prior art system for determining birefringence of a material. FIG. 3 is a schematic of an automated system for measuring an optical property. FIG. 4 is a block diagram of a process for measuring an optical property using the system shown in FIG. FIG. 5 shows how data points are sampled using the process described in FIG. FIG. 6 is a schematic illustration of a process used to generate nodes. FIG. 7 shows the automated system of FIG. 3 at neutral position. FIG. 8 shows data points sampled along orthogonal axes of a sample. FIG. 3 illustrates an automated system The automated system The automated system In operation, a light beam The birefringence at a particular point in the sample FIG. 4 illustrates the process for automatically creating a birefringence map for a sample In addition to prompting the user for information about the geometry of the sample The process continues with a grid generation module The operations of the grid generation module Assuming that the length Y is superimposed on the positive X-axis of the coordinate system shown in FIG. 5, then the coordinates (x,y) of the Z data points or nodes along the positive X-axis would be:
If the length Y is superimposed on the negative X-axis of the coordinate system, then the coordinates of the Z data points measured along the length Y would be:
Taking into account the origin O of the coordinate system and the edges E
where
The expression (3) will be evaluated for every value of y to obtain the coordinates of the nodes The x- and y-components of the coordinates determined using expression (3) above are stored in the first column and second column of the matrix C, respectively. Note that the dimension of the matrix C will be (2Z+3) by 2. The matrix C represents the points on the sample The radial distances R The grid generation module next determines the angular position Φ Note that Φ
The vector ΔΦ may be used in place of the vector Φ to position align nodes with the Y-axis. The vector ΔΦ can be sorted in ascending order. If vector ΔΦ is used and sorted, any sorting applied to the vector ΔΦ should also be applied to corresponding entries in the vector R and the matrix C. The vectors R and Φ (or ΔΦ) and the matrix C are stored on an electromagnetic medium Referring back to FIG. 4, the process continues by moving the translation stages As shown at The next step is to increment i by one, shown at It should be understood that in the process described above, the origin O of the coordinate system does not have to be at the geometric center of the sample For birefringence measurements, it has been found that the sample holder The process can be adapted to make measurements only about orthogonal axes of the sample
and nodes along the Y-axis that are described by expressions:
The measurements along the orthogonal lines would then be made using the same process illustrated in FIG. While the example embodiment described herein is directed to measurement of the birefringence of a sample, it should be clearly understood that the automated system can measure other types of optical properties, e.g., transmission. The process described above can easily be extended to other measurements, simply by changing some of the elements such as the analyzer or light source. It is also possible to perform continuous measurements while keeping spatial resolution constant. Also, the process can be extended to look at any property in which there is an energy source and a detector. Specifically, the process applies to any property that measures the state of the energy entering a sample and compares it to the state of the energy leaving a sample. Those skilled in the art will appreciate that other embodiments of the invention can be devised which do not depart from the spirit of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. Patent Citations
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